Lets say you have Lucia max Skill at 29, S in sword weapon level, equipped with Vague Katti, adjacent to Elincia and Geoffrey, has the Gamble and Wrath skills, and her HP is less than half.

As a Swordmaster, she gets a +15 critical boost. The Vague Katti has 35 critical. Standing next to Elincia and Geoffrey gives her another +15 critical boost. Her Skill at 29 also gives her a +14 in critical. Wrath gives her a +50 critical boost and Gamble doubles her chance for a critical.